I guess it is always a case of keeping Markdown "pure" or adding
support for attributes/classes when it is practical.

In my scenario, I had a customer who did not like to use any HTML in
their input, but still wanted to be able to tell the CMS back-end that
a specific paragraph was a disclaimer, that a specific paragraph
should be centered, that an image should be left or right floated,
etc. So the solution was to use the syntax {disclaimer} after a
paragraph, or {center}, {left}, {right}.

Its a compromise, that stays true to the Markdown syntax, and don't
introduce a new confusing html syntax in the mix.

That was my original motivation.

- Egil

> The rule of thumb I try to follow when doing extensions can be found